This image is comprised
of 12 X-ray images of the sun's atmosphere
taken between 1991 and 1995 in 120-day increments.
This composite image clearly demonstrates
how the sun changes during the solar cycle.
The images were taken from the Yohkoh satellite,
and can be found on the Yohkoh
SXT site.

Scientists became aware that the sun went through
cycles and changes by observing sunspots, the darker,
relatively cooler areas of the sun. The number of
sunspots can be an indication of the degree of solar
activity. The average number of visible sunspots varies
over time, increasing and decreasing on a regular
cycle of between 9.5 to 11 years, on average about
10.8 years. An amateur astronomer, Heinrich Schwabe,
was the first to note this cycle in 1843. The part
of the cycle with low sunspot activity is referred
to as "solar minimum," the portion with
high activity is known as "solar maximum."
The year 2000, it is believed, will be the solar maximum
for the current solar cycle.

Solar Flares and CMEs
While sunspots have been used historically to indicate
levels of solar activity, other solar features increase
in number and intensity along with the fluctuations
in the sun's magnetic field structure. Coronal mass
ejections (CMEs) and emissions of high-energy solar
flares become more common and more intense during
the period of solar maximum. This increase in solar
activity can affect us here on earth (and in earth's
orbit) by what is known as space weather.

This NASA
illustration shows the earth's magnetosphere
and its interaction with the sun.

Effects on Earth
Ordinarily, the earth's own magnetic field protects
it from most of the sun's emissions. But during periods
of intense solar activity geomagnetic storms can produce
heightened, spectacular displays of the Aurora Borealis
and Aurora Australis, known as the Northern and Southern
Lights. Geomagnetic storms can also disrupt radio transmissions
and affect power grids. Energetic electromagnetic bombardments
can interfere with the transmission of radio waves and
the flow of electric current in wires. Radio operators
are familiar with solar maximum and have to deal with
an increase in static on the airwaves. Occasionally,
radio signals can be completely drowned out. Power grids
can be overloaded by these same bombardments. In 1989,
during the last solar maximum, the power grid that supplies
Canada's Quebec province was knocked out by a geomagnetic
storm.

Satellite Disruption
Since satellites are outside the protection of the earth's
atmosphere, they are particularly vulnerable to the
severe geomagnetic storms that can result from solar
activity. According to stellar physicist David Dearborn,
"As the accelerated energetic gas particles from the
sun interact with the earth's magnetic field...they
slide around theearth and form current sheets
that satellites have to deal with. Satellites move from
a region of space that has one charge to an area that
has another charge, and when they cross those boundaries,
the surface of the satellite can suddenly change polarity
(as it moves into a region where there is a different
electric field). You get arcing and you get electric
currents flowing inside the satellite in places where
they're not supposed to flow, and that can be very bad
for the satellites."

In addition to these polarity changes which can damage
sensitive electronics in satellites, the increased solar
emission also causes the earth's atmosphere to "puff
out," creating increased drag on orbiting satellites.
This increased drag can cause satellite orbits to decay
more rapidly than predicted. The 100+ ton Skylab station
is a good example. Launched in 1973, the station was
supposed to remain in orbit until the 1980s. The purpose
of Skylab was, among other things, to study the sun.
Ironically, due to increased solar activity, Skylab
re-entered earth's atmosphere in 1979--raining debris
over the Indian Ocean and parts of Western Australia.

Solar Max 2000
It has yet to be seen whether the upcoming solar maximum
will affect us here on earth (or in earth's orbit) in
the way some of the past maximums have. Scientists still
don't completely understand all of the aspects of the
solar cycle and it's difficult to predict just how strong
the solar maximum will be. Each of the 22 solar cycles
studied since the one Heinrich Schwabe noted in 1843
has varied in intensity. One thing is certain, this
current solar cycle, #23, will be the most closely observed
ever. There will be nearly one dozen space-based observatories
watching the sun during this solar maximum. We'll be
watching, too, as we continually update Solar Max 2000
throughout the year.

Glossary
of Solar Terms

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